1. Field of the Invention
The present invention relates to an apparatus and method of fabricating a long-period fiber grating and more particularly, to an apparatus for fabricating a long-period fiber grating using a string which vibrates with a predetermined period.
2. Description of the Related Art
Typical long-period fiber gratings are elements for coupling a core mode, in which light travels through the core of an optical fiber, to a cladding mode, and thus are well suited to gain-flattening of an erbium doped fiber amplifier (EDFA) since they do not act as a reflective type device.
Long-period fiber gratings can be fabricated by a methods of using an amplitude mask on which a light transmission/non-transmission portion has a predetermined period; by vibrating an optical fiber at a predetermined period; or by a method of applying stress to the optical fiber at a predetermined period.
In the method of using an amplitude mask, the refractive index of the core of an optical fiber, which is sensitive to ultraviolet rays, is periodically varied using an amplitude mask having a predetermined period. That is, to obtain an arbitrary coupling peak wavelength .lambda., the refractive index difference between the core and cladding of an optical fiber is formed by directly projecting an ultraviolet laser onto an amplitude-modulated mask having a grating cycle .LAMBDA., thereby fabricating a long-period fiber grating. Here, the coupling peak wavelength value .lambda. is determined by the following Equation 1: EQU .lambda.=(n.sub.core -n.sub.cladding).LAMBDA. (1)
wherein n.sub.core is the refractive index of the core, n.sub.cladding is the refractive index of the cladding, and .LAMBDA. is the period of an amplitude mask.
A conventional apparatus for fabricating a long-period fiber grating using an amplitude mask includes an amplitude mask mounted on a fiber, and a laser source for irradiating laser light onto the amplitude mask. However, in the method of using an amplitude mask, the period of the amplitude mask is determined according to the intended coupling peak wavelength to be obtained. Therefore, the ultraviolet laser may be collimated with various methods or a dual mask may be applied when the period of the amplitude mask is changed.
The long-period fiber grating can also be fabricated by vibrating an optical fiber or by applying stress using comb-shaped probes. However, these alternative methods have problems in that stability and reproducibility are difficult, and the devices required to fabricate the long-period fiber grating are large.